[54]

Secondly

[Annotation by Darwin:] The greater the heat the faster it will be conducted to produce an apparent effect

All crystal conduct heat with different rapidities in different directions. Some have only two rates of conduction viz: along or perpendicr. to an axis — & these are called uniaxal crystals. Some have 3 rates of conduction, in these directions mutually at right-angles, & these are called biaxal crystals.

All fibrous bodies conduct heat at a different rate along and perpendicular to the fibres. Thus a piece of the trunk of a tree conducts heat at 3 different rates 1st parallel to the trunk

2nd from the centre of

[54v]

outwards, & lastly if the piece be not cut from the heart of the tree there is a third rate along a horizontal line touching one of the rings of growth

The line (a) is supposed to stick out perpendicular to the paper [diagram]

[Annotated diagram]

These vertical lines will represent the fibres parallel to trunk

rings of growth

The dots will be the ends of the rings of growth.

It is Dr. Caufield not Dr. Canfield 'on horns of the antilocapra 234) of Descent of Man 2nd ed pubd in 1874

— I'm so sorry I didn't correct index before it went to press as I cd easily have cut off 10 pp. without losing a reference, but cutting out the 'on's & 'the's & superfluous word.

Yrs afftly

G.H.D